LibWeb: Avoid unnecessary work when element display property changes

`play_or_cancel_animations_after_display_property_change` is called
whenever an element is inserted or removed, or it's display property
changes, but it is only required to run if we actually have animations
to play or cancel.

Reduces time spent in the aforementioned function from ~2% to ~0.03%
when loading https://en.wikipedia.org/wiki/2023_in_American_television

Libraries/LibWeb/Animations/Animatable.cpp

@@ -279,6 +279,19 @@ void Animatable::visit_edges(JS::Cell::Visitor& visitor)
     }
 }
 
+void Animatable::set_has_css_defined_animations()
+{
+    ensure_impl().has_css_defined_animations = true;
+}
+
+bool Animatable::has_css_defined_animations() const
+{
+    if (!m_impl)
+        return false;
+
+    return m_impl->has_css_defined_animations;
+}
+
 HashMap<FlyString, GC::Ref<Animation>>* Animatable::css_defined_animations(Optional<CSS::PseudoElement> pseudo_element)
 {
     auto& impl = ensure_impl();

Libraries/LibWeb/Animations/Animatable.h

@@ -52,6 +52,8 @@ public:
     void associate_with_animation(GC::Ref<Animation>);
     void disassociate_with_animation(GC::Ref<Animation>);
 
+    void set_has_css_defined_animations();
+    bool has_css_defined_animations() const;
     HashMap<FlyString, GC::Ref<Animation>>* css_defined_animations(Optional<CSS::PseudoElement>);
     void add_css_animation(FlyString name, Optional<CSS::PseudoElement>, GC::Ref<Animation>);
     void remove_css_animation(FlyString name, Optional<CSS::PseudoElement>);
@@ -79,6 +81,7 @@ private:
     struct Impl {
         Vector<GC::Ref<Animation>> associated_animations;
         bool is_sorted_by_composite_order { true };
+        bool has_css_defined_animations { false };
 
         mutable Array<OwnPtr<HashMap<FlyString, GC::Ref<Animation>>>, to_underlying(CSS::PseudoElement::KnownPseudoElementCount) + 1> css_defined_animations;
         mutable Array<OwnPtr<Transition>, to_underlying(CSS::PseudoElement::KnownPseudoElementCount) + 1> transitions;

Libraries/LibWeb/CSS/StyleComputer.cpp

@@ -1266,6 +1266,7 @@ void StyleComputer::process_animation_definitions(ComputedProperties const& comp
         }
 
         effect->set_target(&element);
+        element.set_has_css_defined_animations();
         element_animations->set(animation_properties.name, animation);
     }
 

Libraries/LibWeb/DOM/Element.cpp

@@ -4177,6 +4177,10 @@ FlyString const& Element::html_uppercased_qualified_name() const
 
 void Element::play_or_cancel_animations_after_display_property_change()
 {
+    // OPTIMIZATION: We don't care about elements with no CSS defined animations
+    if (!has_css_defined_animations())
+        return;
+
     // OPTIMIZATION: We don't care about animations in disconnected subtrees.
     if (!is_connected())
         return;